The present invention relates to improvements in magnet coil formers. More specifically, the present invention relates to an improved magnetic coil former for use in a magnetic resonance imaging (MRI) system.
It is known to manufacture magnetic coils by winding a superconducting wire of suitable magnetic characteristics around a former. The former may be removed after the coil has been wound around it or it may be left in place.
Furthermore, it is known to use a pair of magnetic coils, often referred to in the art as drive coils, to generate a homogenous primary magnetic field suitable for use in an MRI system.
It is also known to use a pair of compensating magnetic coils which have currents running through them in opposite sense to the currents running through the drive coils. The compensating coils generate a magnetic field which functions to improve the homogeneity of the primary magnetic field.
U.S. Pat. No. 5,402,094 discloses an MRI magnet for mammography, in which the drive coil is mounted within the center of an annular ferromagnetic former, with compensating coils of increasing diameter mounted in groves within the annular former.
FIG. 1 shows a pair of known magnetic coil forms 10a and 10b. The formers are made of a non-ferromagnetic material such as stainless steel. Disposed of former 10a is driver coil 12a and compensating coil 14a. Similarly, disposed on former 10b is driver coil 12b and compensating coil 14b. The formers are arranged in parallel planes and have a common central axis Z. Coils 12a and 12b have larger radii than coils 14a and 14b. As is known in the art, drive coils arranged as such will generate a primary magnetic field {overscore (B)} parallel to the central axis Z. Coils 14a and 14b will generate a compensating magnetic field which will have the well-known effect of improving the homogeneity of the primary magnetic field {overscore (B)}. A patient 35 lies in the space between the formers and in a plane parallel to the formers.
A technical problem exists with known magnetic coil type systems in that they suffer from excessive collapsing hoop stress. This occurs when the force of repulsion between the drive coil and the corresponding compensating coil is so great that the compensating coil requires substantial mechanical support means to prevent it from collapsing towards its central axis.
The collapsing hoop stress can be reduced by increasing the diameter of the drive coil with respect to the compensating coil. However, this has the negative effect of reducing the field of view of the MRI system.
Thus it is an object of the present invention to provide a coil former which reduces the collapsing hoop stress on the compensating coils.
It is a further object of the present invention to provide a coil former which reduces the peak magnetic field in the coils, thereby increasing the maximum allowable current in the coil's superconducting wire. Advantageously, this allows for a reduction in the amount of superconducting wire required to achieve the desired magnetic field strength.
It is yet a further object of the present invention to provide a coil former which facilitates the construction of a more compact MRI system.